1. Field of the Invention
This invention relates to a method and system for epitaxial growth of high purity materials on an atomic or molecular layer by layer basis.
2. Brief Description of the Prior Art
Atomic layer epitaxy (ALE) has been in existence for in excess of ten years as noted by M. A. Harman in Vacuum, volume 42, page 61 (1991) and Atomic Layer Epitaxy by T. Suntola and M. Simpson, Editors, Chapman and Hall (1990) and U.S. Pat. No. 4,048,430 of T. Suntola and M. Antson. ALE has been shown to produce high quality crystalline films of a variety of materials. The ALE approach is, in actuality, a special mode of other physical and chemical deposition growth techniques, such as chemical vapor deposition (CVD) or molecular beam epitaxy (MBE). ALE is based upon chemical reactions at carefully prepared, typically heated, substrate surfaces. The constituent elements of the film are delivered to the sample sequentially as pulses of neutral molecules or atoms.
The chemical reactions in the ALE process are self-limiting in that the available bonds (reactive sites) on the surface are consumed in their entirety. This limits the growth of the film to single layers of the reactant species. Through using the surface chemistry in the process, enhanced reactivity of the precursor may be expected at lower temperatures. The choice of the molecular species is based upon known surface chemistry in order to take advantage of the self-limiting reaction and lower growth temperatures. This includes choosing a precursor molecule on the basis of the reaction and lower growth temperatures. This also includes choosing a precursor molecule on the basis of the steric interactions of an adsorbed/reactant species, which permits an accurate control of surface coverage.
Two basic variants of ALE exist. A first such variant utilizes a direct ALE process whereby elemental constituents are deposited onto the substrate and direct chemical reactions ensue between these reactants and the outermost surface atoms. A second such variant process relies upon the sequential surface exchange reactions between the substrate surface atoms and the molecules of the reactants which are chemical compounds.
Typically, the vacuum chamber used in the ALE approach is backfilled with a gas phase molecule to result in high vacuum pressures on the order of 10.sup.-5 Torr (relatively high pressure). The vacuum chamber is frequently purged with a non-reactive gas between exposures. These relatively high pressures can result in the introduction of impurities into the film due to the usual purging process.
Precision dosing techniques of single crystal surfaces with molecular species have also been understood for some time as noted by C. T. Campbell and S. M. Velone in Journal of Vacuum Science Technology, Vol 43, page 408 (1985) and by A. Winkler and J. T. Yates, Jr. in Journal of Vacuum Science Technology, Vol 46, page 2929 (1988) and have largely concentrated upon academic surface science experiments as noted by R. M. Wallace in Backscattering and Chemical Investigation of Semiconductor Surfaces, a Ph.D. dissertation, University of Pittsburgh (1988). The technique comprises a gas reservoir of high purity gas vapor, typically at pressures below 1 atmosphere. This reservoir is connected to the vacuum chamber used for exposing the substrate through a small conductance limiting orifice, on the order of a few microns in diameter. This permits precise control of the molecular flux into the system by manipulating the reservoir pressure. The flux of molecules is passed to an effusive capillary assembly, generally comprising an array of capillaries, and directed at the substrate.
As noted above, the purging step, which has been required by the prior art, introduces impurities into the system when operating on an atomic and/or molecular scale. It is therefore desirable to eliminate the purging step from the operation.